Managing recharge power for implantable medical devices

ABSTRACT

Devices, systems, and techniques for controlling charging power based on a cumulative thermal dose to a patient are disclosed. Implantable medical devices may include a rechargeable power source that can be transcutaneously charged. An external charging device may calculate an estimated cumulative thermal dose delivered to the patient during charging over a predetermined period of time. Based on the estimated cumulative thermal dose, the external charging device may select a power level for subsequent charging of the rechargeable power source. In one example, the charging device may select a high power level when the cumulative thermal dose has not exceeded a thermal dose threshold and select a low power level when the cumulative thermal dose has exceeded the thermal dose threshold.

TECHNICAL FIELD

The disclosure relates to implantable medical devices and, moreparticularly, rechargeable power supplies for implantable medicaldevices.

BACKGROUND

Implantable medical devices may be used to monitor a patient conditionand/or deliver therapy to the patient. In long term or chronic uses,implantable medical devices may include a rechargeable power source(e.g., comprising one or more capacitors or batteries) that extends theoperational life of the medical device to weeks, months, or even yearsover a non-rechargeable device.

When the energy stored in the rechargeable power source has beendepleted, the patient may use an external charging device to rechargethe power source. Since the rechargeable power source is implanted inthe patient and the charging device is external of the patient, thischarging process may be referred to as transcutaneous charging. In someexamples, transcutaneous charging may be performed via inductivecoupling between a primary coil in the charging device and a secondarycoil in the implantable medical device. When a current is applied to theprimary coil and the primary coil is aligned to the secondary coil,electrical current is induced in the secondary coil within the patient.Therefore, the external charging device does not need to physicallyconnect with the rechargeable power source for charging to occur.

SUMMARY

In general, the disclosure is directed to devices, systems, andtechniques for controlling charging power based on an estimation of acumulative thermal dose delivered to a patient during a charging period.An external charging device may be used to transcutaneously charge arechargeable power source of the IMD. An external charging device maycalculate an estimated cumulative thermal dose delivered to the patientduring the charging process. Based on the calculated cumulative thermaldose, the external charging device may control charging of therechargeable power source. For example, the external charging device mayselect a power level for subsequent charging of the rechargeable powersource.

In one aspect, the disclosure is directed to a method that includescalculating, by a processor, an estimated cumulative thermal dosedelivered to a patient during charging of a rechargeable power source ofan implantable medical device over a period of time and selecting, bythe processor, a power level for subsequent charging of the rechargeablepower source based on the estimated calculated cumulative thermal dose.

In another aspect, the disclosure is directed to a device that includesa processor configured to calculate an estimated cumulative thermal dosedelivered to a patient during charging of a rechargeable power source ofan implantable medical device over a period of time and select a powerlevel for subsequent charging of the rechargeable power source based onthe estimated cumulative thermal dose.

In a further aspect, the disclosure is directed to a computer-readablestorage medium including instructions that cause at least one processorto calculate an estimated cumulative thermal dose delivered to a patientduring charging of a rechargeable power source of an implantable medicaldevice over a period of time and select a power level for subsequentcharging of the rechargeable power source based on the estimatedcumulative thermal dose.

In another further aspect, the disclosure is directed to a system thatincludes an implantable medical device comprising a rechargeable powersource, an external charging device comprising a charging moduleconfigured to charge the rechargeable power source, and at least oneprocessor configured to calculate an estimated cumulative thermal dosedelivered to a patient during charging of the rechargeable power sourceover a period of time and select a power level for subsequent chargingof the rechargeable power source based on the estimated cumulativethermal dose.

The details of one or more example are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram illustrating an example system thatincludes an implantable medical device (IMD) and an external chargingdevice that charges a rechargeable power source of the IMD.

FIG. 2 is a block diagram of the example IMD of FIG. 1.

FIG. 3 is a block diagram of the example external charging device ofFIG. 1.

FIG. 4 is a graph of example temperatures generated in a patient duringIMD recharging over a period of time.

FIGS. 5A and 5B are graphs of example selected power levels for chargingand an associated rechargeable power supply charge level due to theselected power levels.

FIGS. 6A and 6B are graphs of example selected power levels for chargingand an associated rechargeable power supply charge level due to theselected power levels.

FIGS. 7A and 7B are graphs of example selected power levels due to animposed lockout period after charging with a high power level.

FIG. 8 is a flow diagram that illustrates an example technique forselecting a power level for charging an implantable rechargeable powersource based on a calculated cumulative thermal dose.

FIG. 9 is a flow diagram that illustrates an example technique forselecting a power level for charging an implantable rechargeable powersource based on an available cumulative thermal dose remaining for thecharging process.

FIG. 10 is a flow diagram that illustrates an example technique forimplementing a lockout period for high power level charging of arechargeable power source.

DETAILED DESCRIPTION

This disclosure is generally directed to devices, systems, andtechniques for controlling power user to charge a rechargeable powersource based on a cumulative thermal dose delivered to a patient.Implantable medical devices (IMDs) may be implanted within a patient andused to monitor a parameter of the patient and/or deliver a therapy tothe patient. To extend the operational life of the IMD, the IMD mayinclude a rechargeable power source (e.g., one or more capacitors orbatteries). When the rechargeable power source is being recharged, thepower transmitted to the IMD may generate heat that increases thetemperature of the IMD. In order to prevent this increased temperaturefrom damaging patient tissue adjacent to the IMD, charging sessions maybe limited to predetermined durations and/or reduced power levels may beused to recharge the rechargeable power source. However, this approachmay increase recharge durations and/or prevent the rechargeable powersource from being fully charged.

As disclosed herein, an estimate of the cumulative thermal dose (e.g.,an estimated cumulative thermal dose) delivered to the patient duringrecharging may be calculated on a continual basis. The external chargedevice may monitor the cumulative thermal dose and control the chargingpower level of the charging process to limit the heat generated in theIMD. For example, the charging device may select a high power level tocharge the rechargeable power source at a high rate until the calculatedcumulative thermal dose indicates the power level needs to be reduced.The charging device may then terminate charging or select a lower powerlevel to continue charging the rechargeable power source at lower IMDtemperatures.

The cumulative thermal dose feedback may thus reduce the need toconservatively estimate recharge times and recharge power levels to keepthe temperature of the IMD within safe limits. Instead, the chargedevice may charge the rechargeable power source at high rates until thecumulative thermal dose indicates that the temperature, and the chargepower level, needs to be reduced. Lower power levels may be used tocontinue charging the rechargeable power source until the power sourceis fully charged. This closed loop feedback approach may reduce theamount of time needed to charge rechargeable power sources and/orincrease the likelihood that the rechargeable power source is fullycharged after a recharge session.

The cumulative thermal dose may be an indication, or estimation, of thetotal amount of heat to which the tissue surrounding the IMD has beenexposed. Even at temperatures too low to cause immediate tissuenecrosis, elevated temperatures may still be undesirable for thepatient. Therefore, it may be useful to monitor the amount of time thattissue is exposed to elevated temperatures (e.g., temperatures above 39degrees Celsius and below 43 degrees Celsius). This cumulative thermaldose may be used to control the recharge process and uncomfortable andundesirable elevated temperatures. For example, the cumulative thermaldose may be calculated by integrating the tissue temperature over apredetermined period of time. Since the exact thermal dose delivered tothe patient may be difficult to exactly measure, the cumulative thermaldose described herein may be used as an estimation of the actualcumulative thermal dose delivered to the patient. However, thecalculated cumulative thermal dose described herein may be substantiallysimilar to the actual thermal dose received by the patient. The powerlevel used to charge the rechargeable power source may then be selectedbased on the comparison of the cumulative thermal dose to one or morethresholds.

In some examples, the charging device may select power levels withdecreasing intensity as the cumulative thermal dose reaches a threshold.Incrementally decreasing the power level may minimize the risk ofexceeding the cumulative thermal dose threshold from residual heat inthe IMD even after reducing the power level. In other examples, thecharging device may employ a lockout period to prevent frequent use ofhigh power levels during charging. For example, the lockout period maybegin after a high power level has ceased, and a new high power levelmay not be used for charging until after the lockout period has expired.The lockout period may be a predetermined duration of time or determinedbased on the length of time the previous high power level was used tocharge the rechargeable power source.

FIG. 1 is a conceptual diagram illustrating an example system 10 thatincludes an implantable medical device (IMD) 14 and an external chargingdevice 20 that charges a rechargeable power source 18. Although thetechniques described in this disclosure are generally applicable to avariety of medical devices including medical devices such as patientmonitors, electrical stimulators, or drug delivery devices, applicationof such techniques to implantable neurostimualtors will be described forpurposes of illustration. More particularly, the disclosure will referto an implantable neurostimulation system for use in spinal cordstimulation therapy, but without limitation as to other types of medicaldevices.

As shown in FIG. 1, system 10 includes an IMD 14 and external chargingdevice 20 shown in conjunction with a patient 12, who is ordinarily ahuman patient. In the example of FIG. 1, IMD 14 is an implantableelectrical stimulator that delivers neurostimulation therapy to patient12, e.g., for relief of chronic pain or other symptoms. Generally IMD 14may be a chronic electrical stimulator that remains implanted withinpatient 12 for weeks, months, or even years. In the example of FIG. 1,IMD 14 and lead 16 may be directed to delivering spinal cord stimulationtherapy. In other examples, IMD 14 may be a temporary, or trial,stimulator used to screen or evaluate the efficacy of electricalstimulation for chronic therapy. IMD 14 may be implanted in asubcutaneous tissue pocket, within one or more layers of muscle, orother internal location. IMD 14 includes rechargeable power source 18and IMD 14 is coupled to lead 16.

Electrical stimulation energy, which may be constant current or constantvoltage based pulses, for example, is delivered from IMD 14 to one ormore targeted locations within patient 12 via one or more electrodes(not shown) of lead 16. The parameters for a program that controlsdelivery of stimulation energy by IMD 14 may include informationidentifying which electrodes have been selected for delivery ofstimulation according to a stimulation program, the polarities of theselected electrodes, i.e., the electrode configuration for the program,and voltage or current amplitude, pulse rate, pulse shape, and pulsewidth of stimulation delivered by the electrodes. Electrical stimulationmay be delivered in the form of stimulation pulses or continuouswaveforms, for example.

In the example of FIG. 1, lead 16 is disposed within patient 12, e.g.,implanted within patient 12. Lead 16 tunnels through tissue of patient12 from along spinal cord 22 to a subcutaneous tissue pocket or otherinternal location where IMD 14 is disposed. Although lead 16 may be asingle lead, lead 16 may include a lead extension or other segments thatmay aid in implantation or positioning of lead 16. In addition, aproximal end of lead 16 may include a connector (not shown) thatelectrically couples to a header of IMD 14. Although only one lead 16 isshown in FIG. 1, system 10 may include two or more leads, each coupledto IMD 14 and directed to similar or different target tissue sites. Forexample, multiple leads may be disposed along spinal cord 22 or leadsmay be directed to spinal cord 22 and/or other locations within patient12.

Lead 16 may carry one or more electrodes that are placed adjacent to thetarget tissue, e.g., spinal cord 22 for spinal cord stimulation (SCS)therapy. One or more electrodes may be disposed at a distal tip of lead16 and/or at other positions at intermediate points along lead 16, forexample. Electrodes of lead 16 transfer electrical stimulation generatedby an electrical stimulation generator in IMD 14 to tissue of patient12. The electrodes may be electrode pads on a paddle lead, circular(e.g., ring) electrodes surrounding the body of the lead, conformableelectrodes, cuff electrodes, segmented electrodes, or any other type ofelectrodes capable of forming unipolar, bipolar or multipolar electrodeconfigurations for therapy. In general, ring electrodes arranged atdifferent axial positions at the distal ends of lead 16 will bedescribed for purposes of illustration.

In alternative examples, lead 16 may be configured to deliverstimulation energy generated by IMD 14 to stimulate one or more sacralnerves of patient 12, e.g., sacral nerve stimulation (SNS). SNS may beused to treat patients suffering from any number of pelvic floordisorders such as pain, urinary incontinence, fecal incontinence, sexualdysfunction, or other disorders treatable by targeting one or moresacral nerves. Lead 16 and IMD 14 may also be configured to provideother types of electrical stimulation or drug therapy (e.g., with lead16 configured as a catheter). For example, lead 16 may be configured toprovide deep brain stimulation (DBS), peripheral nerve stimulation(PNS), or other deep tissue or superficial types of electricalstimulation. In other examples, lead 16 may provide one or more sensorsconfigured to allow IMD 14 to monitor one or more parameters of patient12. The one or more sensors may be provided in addition to, or in placeof, therapy delivery by lead 16.

IMD 14 delivers electrical stimulation therapy to patient 12 viaselected combinations of electrodes carried by lead 16. The targettissue for the electrical stimulation therapy may be any tissue affectedby electrical stimulation energy, which may be in the form of electricalstimulation pulses or waveforms. In some examples, the target tissueincludes nerves, smooth muscle, and skeletal muscle. In the exampleillustrated by FIG. 1, the target tissue for electrical stimulationdelivered via lead 16 is tissue proximate spinal cord 22 (e.g., one ormore target locations of the dorsal columns or one or more dorsal rootsthat branch from spinal cord 22. Lead 16 may be introduced into spinalcord 22 via any suitable region, such as the thoracic, cervical orlumbar regions. Stimulation of dorsal columns, dorsal roots, and/orperipheral nerves may, for example, prevent pain signals from travelingthrough spinal cord 22 and to the brain of the patient. Patient 12 mayperceive the interruption of pain signals as a reduction in pain and,therefore, efficacious therapy results. For treatment of otherdisorders, lead 16 may be introduced at any exterior location of patient12. In this manner, skin opening 18 may be located at any exterior skinlocation in other examples.

Although lead 16 is described as generally delivering or transmittingelectrical stimulation signals, lead 16 may additionally oralternatively transmit electrical signals from patient 12 to IMD 14 formonitoring. For example, IMD 14 may utilize detected nerve impulses todiagnose the condition of patient 12 or adjust the delivered stimulationtherapy. Lead 16 may thus transmit electrical signals to and frompatient 12.

A user, such as a clinician or patient 12, may interact with a userinterface of an external programmer (not shown) to program IMD 14.Programming of IMD 14 may refer generally to the generation and transferof commands, programs, or other information to control the operation ofIMD 14. For example, the external programmer may transmit programs,parameter adjustments, program selections, group selections, or otherinformation to control the operation of IMD 14, e.g., by wirelesstelemetry or wired connection.

In some cases, an external programmer may be characterized as aphysician or clinician programmer if it is primarily intended for use bya physician or clinician. In other cases, the external programmer may becharacterized as a patient programmer if it is primarily intended foruse by a patient. A patient programmer is generally accessible topatient 12 and, in many cases, may be a portable device that mayaccompany the patient throughout the patient's daily routine. Ingeneral, a physician or clinician programmer may support selection andgeneration of programs by a clinician for use by stimulator 14, whereasa patient programmer may support adjustment and selection of suchprograms by a patient during ordinary use. In other examples, externalcharging device 20 may be included, or part of, an external programmer.In this manner, a user may program and charge IMD 14 using one device,or multiple devices.

IMD 14 may be constructed of any polymer, metal, or composite materialsufficient to house the components of IMD 14 (e.g., componentsillustrated in FIG. 2) within patient 12. In this example, IMD 14 may beconstructed with a biocompatible housing, such as titanium or stainlesssteel, or a polymeric material such as silicone or polyurethane, andsurgically implanted at a site in patient 12 near the pelvis, abdomen,or buttocks. The housing of IMD 14 may be configured to provide ahermetic seal for components, such as rechargeable power source 18. Inaddition, the housing of IMD 14 may be selected of a material thatfacilitates receiving energy to charge rechargeable power source 18.

As described herein, rechargeable power source 18 may be included withinIMD 14. However, in other examples, rechargeable power source 18 couldbe located external to a housing of IMD 14, separately protected fromfluids of patient 12, and electrically coupled to electrical componentsof IMD 14. This type of configuration of IMD 14 and rechargeable powersource 18 may provide implant location flexibility when anatomical spacefor implantable devices is minimal. In any case, rechargeable powersource 18 may provide operational electrical power to one or morecomponents of IMD 14.

Rechargeable power source 18 may include one or more capacitors,batteries, or components (e.g. chemical or electrical energy storagedevices). Example batteries may include lithium-based batteries, nickelmetal-hydride batteries, or other materials. Rechargeable power source18 is also rechargeable. In other words, rechargeable power source 18may be replenished, refilled, or otherwise capable of increasing theamount of energy stored after energy has been depleted. Rechargeablepower source 18 may be subjected to numerous discharge and rechargecycles (e.g., hundreds or even thousands of cycles) over the life ofrechargeable power source 18 in IMD 14. Rechargeable power source 18 maybe recharged when fully depleted or partially depleted.

Charging device 20 may be used to recharge rechargeable power source 18and IMD 14 when implanted in patient 12. Charging device 20 may be ahand-held device, a portable device, or a stationary charging system. Inany case, charging device 20 may include components necessary to chargerechargeable power source 18 through tissue of patient 12. In someexamples, charging device 20 may only perform charging of rechargeablepower source 18. In other examples, charging device 20 may be anexternal programmer or other devices configured to perform additionalfunctions. For example, when embodied as an external programmer,charging device 20 may transmit programming commands to IMD 14 inaddition to charge rechargeable power source 18. In another example,charging device 20 may communicate with IMD 14 to transmit and/orreceive information related to the charging of rechargeable power source18. For example, IMD 14 may transmit temperature information of IMD 14and/or rechargeable power source 18, received power during charging, thecharge level of rechargeable power source 18, charge depletion ratesduring use, or any other information related to power consumption andrecharging of IMD 14 and rechargeable power source 18.

Charging device 20 and IMD 14 may utilize any wireless power transfertechniques that are capable of recharging rechargeable power source 18of IMD 14 when IMD 14 is implanted within patient 14. In one example,system 10 may utilize inductive coupling between a coil of chargingdevice 20 and a coil of IMD 14 coupled to rechargeable power source 18.In inductive coupling, charging device 20 is placed near implanted IMD14 such that a primary coil of charging device 20 is aligned with, i.e.,placed over, a secondary coil of IMD 14. Charging device 20 may thengenerate an electrical current in the primary coil based on a selectedpower level for charging rechargeable power source 18. As describedfurther below, the power level may be selected to control thetemperature of IMD 14 and/or the charge rate of rechargeable powersource 18. When the primary and secondary coils are aligned, theelectrical current in the primary coil may magnetically induce anelectrical current in the secondary coil within IMD 14. Since thesecondary coil is associated with and electrically coupled torechargeable power source 18, the induced electrical current may be usedto increase the voltage, or charge level, of rechargeable power source18. Although inductive coupling is generally described herein, any typeof wireless energy transfer may be used to charge rechargeable powersource 18.

During the energy transfer process that charges rechargeable powersource 18, some of the energy may be converted into heat at rechargeablepower source 18 and/or other components of IMD 14. When increased energylevels are used to charge rechargeable power source 18 at a higher rate,the temperature of IMD 14 may also increase. Although the temperature ofthe IMD 14 housing may not achieve a temperature sufficient to burn ornecrose tissue adjacent to the housing of IMD 14, elevated temperaturesmay be undesirable and uncomfortable over time. Therefore, chargingdevice 20 may control the power levels used to charge rechargeable powersource 18 to reduce or minimize any undesirable temperatures of IMD 14that could be caused by charging rechargeable power source 18. Inaddition, monitoring the temperature of IMD 14 and/or the temperature oftissue adjacent to the housing of IMD 14 may minimize patient discomfortduring the charging process.

In one example, the power level used by charging device 20 to rechargerechargeable power source 18 may be selected or controlled based on acumulative thermal dose delivered to patient 12 by IMD 14. Thecumulative thermal dose may be a metric used to quantify or estimate thetotal temperature exposure to tissue adjacent to IMD 14. As such, thecumulative thermal dose may be an estimated cumulative thermal dose. Inone example, the cumulative thermal dose may be calculated byintegrating the tissue temperature over a period of time. The resultingcumulative thermal dose may be used to equate the delivered heat to acertain tissue temperature level for a certain period of time. Forexample, the clinician may want to limit tissue exposure to heat for 30minutes at 43 degrees Celsius. However, the temperature of IMD 14 willlikely vary from any one temperature over the charging period.Calculation of the cumulative thermal dose may thus allow chargingdevice 20, or IMD 14, to determine when the desired limit to heatexposure is reached even if the actual tissue temperature varies overtime. In other examples, the cumulative thermal dose may be calculatedby adding the average temperature for multiple segments of thepredetermined period of time. In any example, the cumulative thermaldose may be used to determine the total amount of heat or the extent ofelevated temperature exposure for tissue surrounding and/or adjacent toIMD 14.

The tissue temperature used to calculate the cumulative thermal dose maybe determined using several different techniques. Each technique mayresult in a cumulative thermal dose that estimates the actual cumulativethermal dose received by patient 12. However, the estimated cumulativethermal dose calculated by system 10 may be substantially similar to theactual cumulative thermal dose received by patient 12. In one example,the tissue temperature may be measured at one or more locations of IMD14. IMD 14 may include one or more thermocouples, thermistors, or othertemperature sensing elements near the inner surface of the housing ofIMD 14, built within the housing, or disposed on the external of IMD 14.In other examples, IMD 14 may include one or more temperature sensingelements that extend from the outer surface of IMD 14. This directtissue temperature measurement may be the most accurate. However, thetissue temperature measurements may need to be transmitted to chargingdevice 20 such that a processor of charging device 20 can calculate thecumulative thermal dose. Alternatively, a processor of IMD 14 may usethe measured tissue temperature to calculate the cumulative thermaldose. The processor of IMD 14 may then transmit the cumulative thermaldose such that charging device 20 can select the power level, or theprocessor of IMD 14 may directly select power level based on thecumulative thermal dose and instruct charging device 20 on the powerlevel to be used for charging.

In another example, the tissue temperature may be indirectly calculated,or estimated, based on a tissue model and the power transmitted torechargeable power source 18 over a period of time. Charging device 20may monitor the generated current in the primary coil and the resultingpower transmitted from charging device 20 to the secondary coil locatedin IMD 14. The transmitted power may be calculated using the generatedelectrical current, estimated based on the generated electrical currentand expected energy losses due to heat and misalignment, estimated basedon the generated electrical current and energy losses due tomisalignment, or some combination therein. In this manner, chargingdevice 20 may unilaterally determine the tissue temperature.Alternatively, IMD 14 may measure the actual electrical current inducedin the secondary coil coupled to rechargeable power source 18. Based onthis measured current, a processor of IMD 14 may calculate the powertransmitted from charging device 20. IMD 14 may then transmit thecalculated power transmitted from charging device 20 back to chargingdevice 20.

The measured or estimated power transmitted from charging device 20 torechargeable power source 18 may then be applied to a tissue model tocalculate the expected tissue temperature. The tissue model may be oneor more equations that incorporate one or more of the heat capacity oftissue adjacent IMD 14, density of surrounding tissue, inherent bodytemperature, surface are a of the housing of IMD 14, estimated surfacearea of tissue surrounding IMD 14, depth of IMD 14 from the skin ofpatient 12, orientation of the secondary coil within patient 12, or anyother variable that would affect the temperature of surrounding and/orin immediate contact with the housing of IMD 14. The tissue model mayeven be modified over time to account for tissue ingrowth, scar tissue,encapsulation, and other tissue changes due to the biologicalinteraction between the housing of IMD 14 and patient 12. Thetransmitted power may be inputted into the tissue model to calculate anestimation of the tissue temperature as charging device 20 rechargesrechargeable power source 18.

Using the transmitted power techniques, the tissue temperature may becalculated by processors of charging device 20, IMD 14, or somecombination thereof. For example, charging device 20 may unilaterallycalculate the tissue temperature using the tissue model and measuredpower transmitted to IMD 14. In another example, one or more measuredvariables may be communicated from IMD 14 to charging device 20 suchthat charging device can calculate the tissue temperature. IMD 14 maytransmit detected alignment of the primary and secondary coils and/orthe electrical current induced in the secondary coil. In an alternativeembodiment, IMD 14 may measure the transmitted power and calculate thetissue temperature based on that measured power transmitted fromcharging device 20. IMD 14 may then transmit the calculated tissuetemperature to charging device 20, calculate and transmit the cumulativethermal dose to charging device 20 based on the tissue temperature, oreven transmit a selected power level for charging device 20 based on thecalculated cumulative thermal dose. According to these examples, theprocesses needed to determine a tissue temperature (e.g., using ameasured temperature or tissue model calculation) and calculate thecumulative thermal dose may be performed independently by one ofcharging device 20 or IMD 14 or collectively through communicationbetween charging device 20 and IMD 14.

As described herein, information may be transmitted between chargingdevice 20 and IMD 14. Therefore, IMD 14 and charging device 20 maycommunicate via wireless communication using any techniques known in theart. Examples of communication techniques may include, for example, lowfrequency or radiofrequency (RF) telemetry, but other techniques arealso contemplated. In some examples, charging device 20 may include acommunication head that may be placed proximate to the patient's bodynear the IMD 14 implant site in order to improve the quality or securityof communication between IMD 14 and charging device 20. Communicationbetween charging device 20 may occur during or separate from powertransmission.

The cumulative thermal dose is a metric that may reflect the amount ofheat delivered to tissue over a period of time. Since tissue doesdissipate heat, the amount of heat delivered to the tissue does notcontinually compound over the life of patient 12. Instead, the totalamount of delivered heat may only be significant over a certain periodof time. This period of time may be set by the manufacturer or theclinician to a certain number of minutes, hours, or even days.Generally, the period used to calculate the cumulative thermal dose maybe between approximately 10 minutes and 10 days. More specifically, theperiod used to calculate the cumulative thermal dose may be betweenapproximately one hour and 48 hours. In one example, the period may beset to approximately 24 hours. This period may be a rolling period thatextends back from current time. In other words, if the period is 24hours, the cumulative thermal dose may be the total amount ofdegree-minutes in the last 24 hours. In other examples, the period oftime may be represented as an event. For example, the period of time maybe established as a single recharge session (e.g., a continuoustransmission of charging power transmitted from charging device 20 toIMD 14). Therefore, the period may be defined by time or events.

The cumulative thermal dose may be utilized by system 10 to control thepower transmitted from charging device 20 to IMD 14, the rate ofrecharging rechargeable power source 18, and the heat generated by IMD14 during the recharging process. Accordingly, system 10, e.g., one ormore processors of charging device 20 and/or IMD 14, may calculate acumulative thermal dose delivered to patient 12 during charging ofrechargeable power source 18 of IMD 14 over a period of time. The one ormore processors of system 10 may then select a power level forsubsequent charging of the rechargeable power source based on thecalculated cumulative thermal dose. Charging device 20 may then chargerechargeable power source 18 with the selected power level. As discussedin greater detail below, the selected power level may change during thecharging session to control the heat, and cumulative thermal dose,transmitted to tissue surrounding IMD 14. Although a processor of IMD 14may select the charging power level, a processor of charging device 20will be described herein as selecting the charging power level forpurposes of illustration.

In one example, charging device 20 may select a high power level whenthe cumulative thermal dose has not exceeded a thermal dose thresholdand select a low power level when the cumulative thermal dose hasexceeded the thermal dose threshold. In this manner, the high powerlevel may charge rechargeable power source 18 at a high rate to reducecharging time while increasing the temperature of IMD 14. Once thecumulative thermal dose from the elevated IMD 14 temperature exceeds thethermal dose threshold, charging device 20 may select a low power levelto charge rechargeable power source 18 at a slower rate to reduce thetemperature of IMD 14. The low power level may be sufficiently minimalso that any increase in temperature of IMD 14 may have minimal or noeffect on surrounding tissue.

A high power level and a low power level may be subjective and relativeto the charging power that charging device 20 is capable of generatingand transmitting to IMD 14. In some cases, the high power level may bethe maximum power that charging device 20 can generate. This high powerlevel may be referred to as a “boost” or “accelerated” charging levelbecause of the high rate of charge induced in rechargeable power source18. This high rate of charge may minimize the amount of time patient 12needs to recharge rechargeable power source 18. By monitoring thecumulative thermal dose, charging device 20 may charge rechargeablepower source 18 with the high power level for a longer period of timewithout damaging tissue surrounding IMD 14. In other words, merelyestimating the amount of time that charging device 20 can charge at thehigh power level without calculating the actual cumulative thermal dosemay expose tissue to an unsafe level of heat or underutilize the highpower charging, resulting in longer total charge times. Therefore, usingthe cumulative thermal dose delivered to patient 12 may allow system 10to more effectively balance fast charge times and safe heating levels.

In one example, the high power level may be approximately 2.5 Watts andthe low power level may be approximately 1.0 milliwatts (mW). An examplecharge current level may be approximately 100 milliamps (mA) for thehigh power level and approximately 60 mA for the low power level. Thefrequency of the charging signal may be independent of the power level,but the pulse width may generally increase with higher power levelsassuming a constant H-bridge voltages. An H-bridge circuit may be usedas one method to drive the primary coil of charging device 20 with analternating current. An H-bridge circuit may have alternating pairs ofswitches (e.g., transistors) which may be gated on and off using pulses.For example, the width of such pulses may be approximately 4000microseconds (μS) for a high power level and approximately 2000 μS for alow power level with an H-bridge voltage of approximately 10 volts (V).An example primary coil voltage and current for a high power may beapproximately 450 V and approximately 800 mA, respectively, and anexample primary coil voltage and current for a low power level may beapproximately 250 V and approximately 500 mA. These values are merelyexamples, and other examples, may include higher or lower values inaccordance with the techniques described herein.

The thermal dose threshold may be the maximum cumulative thermal doseidentified as still being safe to patient 12. In other words, thethermal dose threshold may be established or selected to prevent tissuefrom being heated to an elevated level and duration that could beuncomfortable or undesirable. The thermal dose threshold may be presetby the manufacturer or selected by a clinician. The thermal dosethreshold may also be modified over time as needed. In some examples,the thermal dose threshold may not be set to the maximum safe dose.Instead, the thermal dose threshold may be set to a lower value toestablish a safety margin below the thermal dose threshold thatminimizes potential overheating of tissue.

The thermal dose threshold may be based on equivalent heating of thetissue at a certain temperature for a predetermined amount of time. Inother words, the thermal dose threshold may be expressed as the totaldegrees over time in elevated temperature. In one example, the thermaldose threshold may be selected as the equivalent to tissue at 43 degreesCelsius for 30 minutes. In another example, the thermal dose thresholdmay be selected as the equivalent to tissue at 43 degrees Celsius for 50minutes. In an alternative example, the thermal dose threshold may beselected as the equivalent to tissue at 41.5 degrees Celsius for 4hours. These thresholds may be summed for comparison to the cumulativethermal dose. For example, tissue at 43 degrees Celsius for 30 minutesmay be expressed by a single value after summing or integrating thetissue temperature elevation (e.g., the difference between 43 degreesCelsius and normal body temperature of 37 degrees Celsius) over the timelimit. When the cumulative thermal dose is calculated in a similarmanner, the cumulative thermal dose may be compared to the thermal dosethreshold as charging device 20 recharges rechargeable power source 18.

The cumulative thermal dose may be calculated by the following equation(1).

$\begin{matrix}{{{CEM}\; 43} = {\sum\limits_{i = 1}^{N}{R^{({43 - T})}t_{i}}}} & (1)\end{matrix}$

“CEM 43” may refer to the cumulative equivalent minutes at 43 degreesCelsius for constant temperature epochs (e.g., reference data). T_(i) isthe measured temperature in degrees Celsius, and t_(i) is the durationof time in minutes. R is a characterizing parameter, or constant, thatmay be set to 0.25 for temperatures less than 43 degrees Celsius. Thevalue of R may be determined experimentally based on known cell and/ortissue characteristics, and R may be a different value in otherexamples. In one example, a CEM 43 limit of 5 minutes may be used ascumulative thermal dose threshold and the power level may be chosen suchthat the cumulative thermal dose of the recharge session may remainlower than the set cumulative thermal dose threshold. In one example,the tissue temperature may be limited to 42 degrees Celsius for theentire recharge session by selecting a certain power level of charging,and the thermal dose threshold would be reached in 20 minutes (e.g.,(0.25̂(43−42)*20=5 minutes)). Incorporating rising and fallingtemperatures over time that occur when charging may be taken into effect(e.g., integrating temperature over time) to allow for longer rechargedurations than would be possible by estimating a constant temperature atany particular power level.

Although charging device 20 may select between two power levels based onthe cumulative thermal dose, charging device 20 may select between threeor more discrete power levels or select the power level from a continualrange of available power levels. For example, charging device 20 mayselect between a high, medium, low, and zero (e.g., no transmittedpower) power levels to minimize charging times and minimizeuncomfortable or undesirable temperatures in surrounding tissue. Inanother example, charging device 20 may continually adjust the powerlevel in small increments, where the increments are established by theavailable resolution of the current able to be generated in the primarycoil of charging device 20. Therefore, these more adjustable powerlevels may result in a power level curve over time as opposed toindividual steps in power levels that would be present using only highand low power levels. In any example, the transmitted power fromcharging device 20 to IMD 14 may be varied based on the calculatedcumulative thermal dose.

In another example, charging device 20 may select a zero power levelwhen the cumulative thermal dose has exceeded the thermal dosethreshold. This zero power level would stop charging rechargeable powersource 18 because charging device 20 would terminate current to theprimary coil in response to the selection of the zero power level.Although low power levels may be used to charge rechargeable powersource 18 at low rates (e.g., a trickle charge), terminating chargingwith the zero power level may allow IMD 14 to cool down at the fastestrate and minimize any additional heating of the tissue surrounding IMD14. In addition, the zero power level may be selected when rechargeablepower source 18 has been fully charged.

In an additional example, charging device 20 may reduce charging powerlevels in anticipation of meeting or exceeding the thermal dosethreshold. Charging device 20 may calculate an available thermal dose bysubtracting the cumulative thermal dose from the thermal dose threshold.In other words, the available thermal dose may be the thermal doseremaining before the cumulative thermal dose exceeds the thermal dosethreshold. This available thermal dose may be used to reduce powerlevels of charging prior to exceeding the thermal dose threshold. Theavailable thermal dose may be compared to a high power dose requirementthat indicates the power should be reduced because the cumulativethermal dose is approaching the thermal dose threshold. The high powerdose requirement may be set to a percentage of the thermal dosethreshold, e.g., between 70 percent and 95 percent of the thermal dosethreshold. The high power dose requirement may instead be set to acertain absolute value below the thermal dose threshold. Using theseguidelines, charging device 20 may select a high power level when theavailable thermal dose is greater than the high power dose requirement.Charging device 20 may then select a low power level when the availablethermal dose is less than the high power dose requirement. Chargingdevice 20 may subsequently continue to charge rechargeable power source18 with the low power level or even terminate charging once thecumulative thermal dose exceeds the thermal dose threshold.

In other examples, system 10 may employ a lockout period that limits thetime charging device 20 can charge rechargeable power source 18 with thehigh power level. The lockout period may begin after a high power levelis used to charge rechargeable power source 18, and the high power levelmay only be used again once the lockout period expires or elapses. Inthis manner, charging device 20 may initiate the lockout period aftercharging rechargeable power source 18 with the high power level suchthat the lockout period prevents selection of the high power level. Theduration of the lockout period may be based on a previous charging timewith the high power level. In other words, the lockout period may be setto a longer period of time when the high power level was used for alonger period of time. In other examples, the lockout period may be setto a single time period regardless of how long charging was performedwith the high power level.

In some examples, IMD 14 may directly adjusting the power level forcharging (e.g., limit the charge current) instead of relying on a changein power level at charging device 20. For example, IMD 14 may employ acircuit that may change from full-wave rectification to half-waverectification to reduce the charge rate and temperature of IMD 14 duringcharging. In other words, IMD 14 may utilize half-wave rectification asa means to reduce the electrical current delivered to rechargeable powersupply 18 instead of reducing the overall power received by IMD 14.Alternatively, IMD 14 may employ other mechanisms such as current and/orvoltage limiters that may limit the charging rate of rechargeable powersupply 18.

Although an implantable rechargeable power source 18 is generallydescribed herein, techniques of this disclosure may also be applicableto a rechargeable power source 18 that is not implanted. For example,rechargeable power source 18 may be external to the skin of patient 12and in physical contact with the skin. Therefore, charging device 20 maycontrol the charging of rechargeable power source 18 with the calculatedcumulative thermal dose even when the power source is external topatient 12. However, tissue models and thresholds may be modified toconfigure charging device 20 for external charging use.

FIG. 2 is a block diagram illustrating example components of IMD 14. Inthe example of FIG. 2, IMD 14 includes temperature sensor 39, coil 40,processor 30, therapy module 34, recharge module 38, memory 32,telemetry module 36, and rechargeable power source 18. In otherexamples, IMD 14 may include a greater or fewer number of components.For example, in some examples, such as examples in which the tissuetemperature is calculated from the transmitted power, IMD 14 may notinclude temperature sensor 39.

In general, IMD 14 may comprise any suitable arrangement of hardware,alone or in combination with software and/or firmware, to perform thevarious techniques described herein attributed to IMD 14 and processor30. In various examples, IMD 14 may include one or more processors 30,such as one or more microprocessors, digital signal processors (DSPs),application specific integrated circuits (ASICs), field programmablegate arrays (FPGAs), or any other equivalent integrated or discretelogic circuitry, as well as any combinations of such components. IMD 14also, in various examples, may include a memory 32, such as randomaccess memory (RAM), read only memory (ROM), programmable read onlymemory (PROM), erasable programmable read only memory (EPROM),electronically erasable programmable read only memory (EEPROM), flashmemory, comprising executable instructions for causing the one or moreprocessors to perform the actions attributed to them. Moreover, althoughprocessor 30, therapy module 34, recharge module 38, and telemetrymodule 36 are described as separate modules, in some examples, processor30, therapy module 34, recharge module 38, and telemetry module 36 arefunctionally integrated. In some examples, processor 30, therapy module34, recharge module 38, and telemetry module 36 correspond to individualhardware units, such as ASICs, DSPs, FPGAs, or other hardware units.

Memory 32 may store therapy programs or other instructions that specifytherapy parameter values for the therapy provided by therapy module 34and IMD 14. In some examples, memory 32 may also store temperature datafrom temperature sensor 39, instructions for recharging rechargeablepower source 18, tissue models, thresholds, instructions forcommunication between IMD 14 and charging device 20, or any otherinstructions required to perform tasks attributed to IMD 14. In thismanner, memory 32 may be configured to store a tissue model such thatprocessor 30 may be configured to calculate the tissue temperaturesurrounding IMD 14 based on the tissue model and power received bysecondary coil 40 and rechargeable power source 18 over a period oftime.

Generally, therapy module 34 may generate and deliver electricalstimulation under the control of processor 30. In some examples,processor 30 controls therapy module 34 by accessing memory 32 toselectively access and load at least one of the stimulation programs totherapy module 34. For example, in operation, processor 30 may accessmemory 32 to load one of the stimulation programs to therapy module 34.In such examples, relevant stimulation parameters may include a voltageamplitude, a current amplitude, a pulse rate, a pulse width, a dutycycle, or the combination of electrodes 17A, 17B, 17C, and 17D thattherapy module 34 uses to deliver the electrical stimulation signal.Although therapy module 34 may be configured to generate and deliverelectrical stimulation therapy via one or more of electrodes 17A, 17B,17C, and 17D of lead 16, therapy module 34 may be configured to providedifferent therapy to patient 12. For example therapy module 34 may beconfigured to deliver drug delivery therapy via a catheter. These andother therapies may be provided by IMD 14.

IMD also includes components to receive power from charging device 20 torecharge rechargeable power source 18 when rechargeable power source 18has been at least partially depleted. As shown in FIG. 2, IMD 14includes secondary coil 40 and recharge module 38 coupled torechargeable power source 18. Recharge module 38 may be configured tocharge rechargeable power source 18 with the selected power leveldetermined by either processor 30 or charging device 20. Althoughprocessor 30 may provide some commands to recharge module 38 in someexamples, processor 30 may not need to control any aspect of recharging.

Secondary coil 40 may include a coil of wire or other device capable ofinductive coupling with a primary coil disposed external to patient 12.Although primary coil 48 is illustrated as a simple loop of in FIG. 3,primary coil 48 may include multiple turns of wire. Secondary coil mayinclude a winding of wire configured such that an electrical current canbe induced within secondary coil 40 from a magnetic field. The inducedelectrical current may then be used to recharge rechargeable powersource 18. In this manner, the electrical current may be induced insecondary coil 40 associated with rechargeable power source 18. Theinduction may be caused by electrical current generated in the primarycoil of charging device 20 and based on the selected power level. Thecoupling between secondary coil 40 and the primary coil of chargingdevice 20 may be dependent upon the alignment of the two coils.Generally, the coupling efficiency increases when the two coils share acommon axis and are in close proximity to each other. Charging device 20and/or IMD 14 may provide one or more audible tones or visualindications of the alignment.

Although inductive coupling is generally described as the method forrecharging rechargeable power source 18, other wireless energy transfertechniques may alternatively be used. Any of these techniques maygenerate heat in IMD 14 such that the charging process can be controlledusing the calculated cumulative thermal dose as feedback.

Recharge module 38 may include one or more circuits that filter and/ortransform the electrical signal induced in secondary coil to anelectrical signal capable of recharging rechargeable power source 18.For example, in alternating current induction, recharge module 38 mayinclude a half-wave rectifier circuit and/or a full-wave rectifiercircuit configured to convert alternating current from the induction toa direct current for rechargeable power source 18. The full-waverectifier circuit may be more efficient at converting the induced energyfor rechargeable power source 18. However, a half-wave rectifier circuitmay be used to store energy in rechargeable power source 18 at a slowerrate. In some examples, recharge module 38 may include both a full-waverectifier circuit and a half-wave rectifier circuit such that rechargemodule 38 may switch between each circuit to control the charging rateof rechargeable power source 18 and temperature of IMD 14.

In some examples, recharge module 38 may include a measurement circuitconfigured to measure the current and/or voltage induced duringinductive coupling. This measurement may be used to measure or calculatethe power transmitted to IMD 14 from charging device 20. In someexamples, the transmitted power may be used to approximate thetemperature of IMD 14 and that of the surrounding tissue. This methodmay be used to indirectly measure the temperature of tissue in contactwith the housing of IMD 14. In other examples, IMD 14 may estimate thetransmitted power using the measured voltage or current after rechargemodule 38 or the charging rate of rechargeable power source 18.

Rechargeable power source 18 may include one or more capacitors,batteries, or other energy storage devices. Rechargeable power source 18may then deliver operating power to the components of IMD 14. In someexamples, rechargeable power source 18 may include a power generationcircuit to produce the operating power. Rechargeable power source 18 maybe configured to operate through hundreds or thousands of discharge andrecharge cycles. Rechargeable power source 18 may also be configured toprovide operational power to IMD 14 during the recharge process. In someexamples, rechargeable power source 18 may be constructed with materialsto reduce the amount of heat generated during charging. In otherexamples, IMD 14 may be constructed of materials that may help dissipategenerated heat at rechargeable power source 18, recharge module 38,and/or secondary coil 40 over a larger surface area of the housing ofIMD 14.

Although rechargeable power source 18, recharge module 38, and secondarycoil 40 are shown as contained within the housing of IMD 14, at leastone of these components may be disposed outside of the housing. Forexample, secondary coil 40 may be disposed outside of the housing of IMD14 to facilitate better coupling between secondary coil 40 and theprimary coil of charging device 20. These different configurations ofIMD 14 components may allow IMD 14 to be implanted in differentanatomical spaces or facilitate better inductive coupling alignmentbetween the primary and secondary coils.

IMD 14 may also include temperature sensor 39. Temperature sensor 39 mayinclude one or more temperature sensors (e.g., thermocouples orthermistors) configured to measure the temperature of IMD 14.Temperature sensor 39 may be disposed internal of the housing of IMD 14,contacting the housing, formed as a part of the housing, or disposedexternal of the housing. As described herein, temperature sensor 39 maybe used to directly measure the temperature of IMD 14 and/or tissuesurrounding and/or contacting the housing of IMD 14. Processor 30, orcharging device 20, may use this temperature measurement as the tissuetemperature feedback to determine the cumulative thermal dose providedto tissue during charging of rechargeable power source 18. Although asingle temperature sensor may be adequate, multiple temperature sensorsmay provide a better temperature gradient or average temperature of IMD14. The various temperatures of IMD 14 may also be modeled and providedto determine the cumulative thermal dose. Although processor 30 maycontinually measure temperature using temperature sensor 39, processor30 may conserve energy by only measuring temperature during rechargesessions. Further, temperature may be sampled at a rate necessary tocalculate the cumulative thermal dose, but the sampling rate may bereduced to conserve power as appropriate.

Processor 30 may also control the exchange of information with chargingdevice 20 and/or an external programmer using telemetry module 36.Telemetry module 36 may be configured for wireless communication usingradio frequency protocols or inductive communication protocols.Telemetry module 36 may include one or more antennas configured tocommunicate with charging device 20, for example. Processor 30 maytransmit operational information and receive therapy programs or therapyparameter adjustments via telemetry module 36. Also, in some examples,IMD 14 may communicate with other implanted devices, such asstimulators, control devices, or sensors, via telemetry module 36. Inaddition, telemetry module 36 may be configured to transmit the measuredtissue temperatures from temperature sensor 39, for example. In someexamples, the tissue temperature may be measured adjacent torechargeable power source 18. In this manner, charging device 20 maycalculate the cumulative thermal dose using the transmitted tissuetemperature. In other examples, processor 30 may calculate thecumulative thermal dose and transmit the calculated cumulative thermaldose using telemetry module 36.

In other examples, processor 30 may transmit additional information tocharging device 20 related to the operation of rechargeable power source18. For example, processor 30 may use telemetry module 36 to transmitindications that rechargeable power source 18 is completely charged,rechargeable power source 18 is fully discharged, or any other chargestatus of rechargeable power source 18. Processor 30 may also transmitinformation to charging device 20 that indicates any problems or errorswith rechargeable power source 18 that may prevent rechargeable powersource 18 from providing operational power to the components of IMD 14.

FIG. 3 is a block diagram of the example external charging device 20.While charging device 20 may generally be described as a hand-helddevice, charging device 20 may be a larger portable device or a morestationary device. In addition, in other examples, charging device 20may be included as part of an external programmer or includefunctionality of an external programmer. In addition, charging device 20may be configured to communicate with an external programmer. Asillustrated in FIG. 3, charging device 20 may include a processor 50,memory 52, user interface 54, telemetry module 56, power module 58, coil48, and power source 60. Memory 52 may store instructions that, whenexecuted by processor 50, cause processor 50 and external chargingdevice 20 to provide the functionality ascribed to external chargingdevice 20 throughout this disclosure.

In general, charging device 20 comprises any suitable arrangement ofhardware, alone or in combination with software and/or firmware, toperform the techniques attributed to charging device 20, and processor50, user interface 54, telemetry module 56, and charging module 58 ofcharging device 20. In various examples, charging device 20 may includeone or more processors, such as one or more microprocessors, DSPs,ASICs, FPGAs, or any other equivalent integrated or discrete logiccircuitry, as well as any combinations of such components. Chargingdevice 20 also, in various examples, may include a memory 52, such asRAM, ROM, PROM, EPROM, EEPROM, flash memory, a hard disk, a CD-ROM,comprising executable instructions for causing the one or moreprocessors to perform the actions attributed to them. Moreover, althoughprocessor 50 and telemetry module 56 are described as separate modules,in some examples, processor 50 and telemetry module 56 are functionallyintegrated. In some examples, processor 50 and telemetry module 56 andcharging module 58 correspond to individual hardware units, such asASICs, DSPs, FPGAs, or other hardware units.

Memory 52 may store instructions that, when executed by processor 50,cause processor 50 and charging device 20 to provide the functionalityascribed to charging device 20 throughout this disclosure. For examplememory 52 may include instructions that cause processor 50 to calculatecumulative thermal doses, establish thresholds, select power levelsbased on the cumulative thermal doses and otherwise control chargingmodule 58, communicate with IMD 14, or instructions for any otherfunctionality. In addition, memory 52 may include a record of selectedpower levels, calculated cumulative thermal doses, or any other datarelated to charging rechargeable power source 18. Processor 50 may, whenrequested, transmit any of this stored data in memory 52 to anothercomputing device for review or further processing.

In some examples, memory 52 may be configured to store datarepresentative of a tissue model used by processor 50 to calculate thetissue temperature based on the tissue model and power transmitted torechargeable power source 18 over a period of time. The tissue model mayindicate how temperate of tissue surrounding IMD 14 changes over timebased on, i.e., as a function of, power received from primary coil 48.Therefore, processor 50 may be able to estimate the tissue temperaturewithout direct measurement of the temperature of tissue surrounding thehousing of IMD 14.

User interface 54 may include a button or keypad, lights, a speaker forvoice commands, a display, such as a liquid crystal (LCD),light-emitting diode (LED), or cathode ray tube (CRT). In some examplesthe display may be a touch screen. As discussed in this disclosure,processor 50 may present and receive information relating to thecharging of rechargeable power source 18 via user interface 54. Forexample, user interface 54 may indicate when charging is occurring,quality of the alignment between coils 40 and 48, the selected powerlevel, current charge level of rechargeable power source 18, duration ofthe current recharge session, anticipated remaining time of the chargingsession, or any other information. Processor 50 may receive some of theinformation displayed on user interface 54 from IMD 14 in some examples.

User interface 54 may also receive user input via user interface 54. Theinput may be, for example, in the form of pressing a button on a keypador selecting an icon from a touch screen. The input may request startingor stopping a recharge session, a desired level of charging, or one ormore statistics related to charging rechargeable power source 18 (e.g.,the cumulative thermal dose). In this manner, user interface 54 mayallow the user to view information related to the charging ofrechargeable power source 18 and/or receive charging commands.

Charging device 20 also includes components to transmit power torecharge rechargeable power source 18 associated with IMD 14. As shownin FIG. 3, charging device 20 includes primary coil 48 and chargingmodule 58 coupled to power source 60. Charging module 58 may beconfigured to generate an electrical current in primary coil 48 fromvoltage stored in power source 60. Although primary coil 48 isillustrated as a simple loop in FIG. 3, primary coil 48 may includemultiple turns of wire. Charging module 58 may generate the electricalcurrent according to a power level selected by processor 50 based on thecumulative thermal dose. As described herein, processor 50 may select ahigh power level, low power level, or a variety of different powerlevels to control the rate of recharge in rechargeable power source 18and the temperature of IMD 14. In some examples, processor 50 maycontrol charging module 58 based on a power level selected by processor30 of IMD 14.

Primary coil 48 may include a coil of wire, e.g., having multiple turns,or other device capable of inductive coupling with a secondary coil 40disposed within patient 12. Primary coil 48 may include a winding ofwire configured such that an electrical current generated within primarycoil 48 can produce a magnetic field configured to induce an electricalcurrent within secondary coil 40. The induced electrical current maythen be used to recharge rechargeable power source 18. In this manner,the electrical current may be induced in secondary coil 40 associatedwith rechargeable power source 18. The coupling efficiency betweensecondary coil 40 and primary coil 48 of charging device 20 may bedependent upon the alignment of the two coils. Generally, the couplingefficiency increases when the two coils share a common axis and are inclose proximity to each other. User interface 54 of charging device 20may provide one or more audible tones or visual indications of thealignment.

Charging module 58 may include one or more circuits that generate anelectrical signal, and an electrical current, within primary coil 48.Charging module 58 may generate an alternating current of specifiedamplitude and frequency in some examples. In other examples, chargingmodule 58 may generate a direct current. In any case, charging module 58may be capable of generating electrical signals, and subsequent magneticfields, to transmit various levels of power to IMD 14. In this mannercharging module 58 may be configured to charge rechargeable power source18 of IMD 14 with the selected power level.

The power level that charging module 58 selects for charging may be usedto vary one or more parameters of the electrical signal generated forcoil 48. For example, the selected power level may specify a wattage,electrical current of primary coil 48 or secondary coil 40, currentamplitude, voltage amplitude, pulse rate, pulse width, or any otherparameter that may be used to modulate the power transmitted from coil48. In this manner, each power level may include a specific parameterset that specifies the signal for each power level. Changing from onepower level to another power level, e.g., a high power level to a lowpower level, may include adjusting one or more parameters. Theparameters of each power level may be selected based on hardwarecharacteristics of charging device 20 and/or IMD 14.

Power source 60 may deliver operating power to the components ofcharging device 20. Power source 60 may also deliver the operating powerto drive primary coil 48 during the charging process. Power source 60may include a battery and a power generation circuit to produce theoperating power. In some examples, the battery may be rechargeable toallow extended portable operation. In other examples, power source 60may draw power from a wired voltage source such as a consumer orcommercial power outlet.

Although power source 60, charging module 58 are shown within a housingof charging device 20 and primary coil 48 is shown external to chargingdevice 20, different configurations may also be used. For example,primary coil 48 may also be disposed within the housing of chargingdevice 20. In another example, power source 60, charging module 58, andprimary coil 48 may be all located external to the housing of chargingdevice 20 and coupled to charging device 20.

Telemetry module 56 supports wireless communication between IMD 14 andcharging device 20 under the control of processor 50. Telemetry module56 may also be configured to communicate with another computing devicevia wireless communication techniques, or direct communication through awired connection. In some examples, telemetry module 56 may besubstantially similar to telemetry module 36 of IMD 14 described herein,providing wireless communication via an RF or proximal inductive medium.In some examples, telemetry module 56 may include an antenna, which maytake on a variety of forms, such as an internal or external antenna.

Examples of local wireless communication techniques that may be employedto facilitate communication between charging device 20 and IMD 14include RF communication according to the 802.11 or Bluetoothspecification sets or other standard or proprietary telemetry protocols.In this manner, other external devices may be capable of communicatingwith charging device 20 without needing to establish a secure wirelessconnection. As described herein, telemetry module 56 may be configuredto receive a measured tissue temperature from IMD 14. The tissuetemperature may be measured adjacent to rechargeable power source 18,such as near the housing of IMD 14 or external of the housing. AlthoughIMD 14 may measure the tissue temperature, one or more differentimplantable temperature sensors (e.g., standalone implantabletemperature sensing devices) may independently measure the tissuetemperature at different positions and transmit the temperature tocharging device 20. In some examples, multiple temperature readings byIMD 14 may be averaged or otherwise used to produce a single temperaturevalue that is transmitted to charging device 20. The temperature may besampled and/or transmitted at different rates, e.g., on the order ofmicroseconds, milliseconds, seconds, minutes, or even hours. Processor50 may then use the received tissue temperature to calculate thecumulative thermal dose.

FIG. 4 is a graph 62 of example temperatures generated in a patientduring IMD recharging over a period of time. As shown in FIG. 4, graph62 includes temperature 64 over time during recharging of rechargeablepower source 18. This temperature may be measured within IMD 14, on thehousing of IMD 14, or within tissue surrounding IMD 14. Alternatively,the temperature may be calculated based on power transmitted to IMD 14and a tissue model of how tissue would respond based on the powertransmitted over time. Therefore, temperature 64 may be representativeof how temperatures in tissue surrounding and/or contacting the housingof IMD 14 may change when rechargeable power source 18 is beingrecharged with given levels of recharge power.

Graph 62 may indicate how temperature 64 changes when charging device 20initially selects a high power level for charging and changes to a lowpower level once the cumulative thermal dose has been reached. Oncecharging of rechargeable power source 18 begins at the zero minute mark(power level change 66), temperature 64 begins to increase fromapproximately 37 degrees Celsius. Because charging device 20 transmitspower at a high power level, rechargeable power source 18 may charge ata fast rate and the temperature of IMD 14 and surrounding tissue mayincrease at a relatively high rates as compared to slower charging rateswith lower transmitted power levels. Temperature 64 may level out at acertain magnitude based on the transmitted power and the ability of thetissue to dissipate heat.

Time T may indicate the amount of time that it takes for the cumulativethermal dose to reach the thermal dose threshold. The cumulative thermaldose may be determined to be representative of the total amount of heattissue has been exposed to over a period of time. The cumulative thermaldose may be calculated using a variety of different techniques thatindicate this total amount of heat. For example, temperature 64 may beintegrated over time to calculate the cumulative thermal dose indegree-minutes. Cumulative thermal dose 70, e.g., the area under thecurve of temperature 62, would thus be representative of the totalamount of heat delivered to tissue from IMD 14 over time. Since thenormal physiological temperature of tissue is approximately 37 degreesCelsius, temperature 64 may only be integrated for temperatures aboutthis 37 degree Celsius floor. However, the cumulative thermal dose maybe calculated using any temperature as a floor as long as the thermaldose threshold, or any other thresholds, are established using thisfloor temperature as well.

In other examples, the cumulative thermal dose may be calculated usingalternative techniques. For example, charging device 20 may averagetemperature 64 for each segment of time (e.g., each minute) and sum theaverage temperatures for each minute to calculate the cumulative thermaldose. Alternatively, the cumulative thermal dose may be calculated usingmore complex equations, such as those disclosed herein, that may accountfor the effect to tissue at different magnitude of temperatures, e.g.,weight time differently at different temperatures. As temperature 64increases, the effects of each incremental change in temperature maycause a disproportional increase in undesirable tissue effects anddecrease patient comfort. In other words, each degree change mayexponentially decrease the amount of time tissue can safely be exposedto that temperature. For example, it may be safe to expose tissue to 41degrees Celsius for 4 hours, but a small increase in temperature to 43degrees may decrease the safe exposure time to only 30 minutes. In thismanner, the cumulative thermal dose may be calculated to account for thenon-linear relationship between temperature and undesirable side effectsover time.

Once the cumulative thermal dose exceeds the thermal dose threshold,charging device 20 may decrease the charging power to a low power levelat power level change 68. In the example of FIG. 4, the cumulativethermal dose exceeded the thermal dose threshold at approximately 35minutes after beginning to charge rechargeable power source 18 with thehigh power level. The low power level may thus decrease the rate thatrechargeable power source 18 is charged and temperature 64 may decreasewith this decreased transmitted power. In other examples, chargingdevice 20 may select the low power level before the thermal dosethreshold is reached and terminate charging once the thermal dosethreshold is reached. In any case, charging device 20 may select thepower level for charging rechargeable power source 18 based on thecumulative thermal dose calculated using temperature 64.

Temperature 64 of graph 62 is only an example of tissue temperaturechanges due to charging rechargeable power source 18 in IMD 14. In theexample of FIG. 4, temperature 64 may increase to approximately 41.5degrees Celsius prior to reducing the power level for charging. In otherexamples, temperature 64 may change at faster or slower rates. Inaddition, temperature 64 may plateau at lower temperatures, plateau athigher temperatures, or not plateau at all during the recharge session.In some examples, temperature 64 may reach temperatures in excess of 42degrees Celsius or even 43 degrees Celsius. In this manner, the thermaldose threshold, method of calculating the cumulative thermal dose, andother variables for managing the cumulative thermal dose received bypatient 12 may be adjusted based on the specific characteristics ofcharging device 20, IMD 14, and even patient 14.

FIGS. 5A and 5B are graphs of example selected power levels for chargingand an associated charge level for rechargeable power supply charge 18due to the selected power levels. Graphs 72 and 78 of FIGS. 5A and 5Bmay correspond to the changes in temperature illustrated in FIG. 4. Inother words, temperature 64 of FIG. 4 is representative of tissuetemperature changes due to power levels selected in FIG. 5A and thecharging rates of FIG. 5B.

As shown in FIG. 5A, graph 72 illustrates example selected power level74 of charging device 20 for charging rechargeable power source 18. Whencharging is initiated, or started, at the zero minute mark, chargingdevice 20 may select a high power level. The initial high power level 74may be selected to charge rechargeable power source 18 at a fast rate,e.g., a “boost”. This fast rate may minimize the amount of time patient12 may need to recharge rechargeable power source 18. Charging device 20may use the high power level to transmit energy to IMD 14 until thecumulative thermal dose exceeds the thermal dose threshold.

Charge level change 76 indicates a change from the selected high powerlevel to the low power level. Charging device 20 may select the lowpower level at charge level change 76 because the cumulative thermaldose exceeded the thermal dose threshold. Then, charging device 20 maycontinue to charge rechargeable power source 18 with the low power leveluntil rechargeable power source 18 is fully charged. Once rechargeablepower source 18 is fully charged, charging device 20 may terminatecharging by selecting a zero power level.

Graph 72 indicates high, medium, and low power levels. Although graph 72indicates that only the high and low power levels are selected, chargingdevice 20 may select the medium power level, or any other power level,in other examples in which various power levels are selected based onthe calculated cumulative thermal dose. In other examples, chargingdevice 20 may only select between high and low power levels whencharging rechargeable power source 18.

As shown in FIG. 5B, graph 78 illustrates charging rate 80 over time dueto varying power levels selected by charging device 20. High rate 82 maybe representative of the charging rate of rechargeable power source 18when charging device 20 selects the high power level for charging. Oncethe cumulative thermal dose exceeds the thermal dose threshold, chargerate change 86 indicates that the charge rate has been lowered. Aftercharge rate change 86, the low power level induces charging rechargeablepower source 18 with low rate 84. Once the charge level for rechargeablepower source 18 reaches approximately 100 percent, the charge rate maybe reduced to zero because the recharge session may be terminated. Inother examples, the low power level may be selected prior to exceedingthe thermal dose threshold to better control the exact cumulativethermal dose delivered to patient 12 when the cumulative thermal doseapproaches the thermal dose threshold.

FIGS. 6A and 6B are graphs of example selected power levels for chargingand the charge level of associated rechargeable power source 18 due toselected power levels. Graphs 90 and 98 of FIGS. 6A and 6B illustratepower level changes alternative to those of FIGS. 5A and 5B. FIG. 6Aillustrates three different power levels for charging and FIG. 6Billustrates the charge rate due to each selected power level. Thetechnique of FIGS. 6A and 6B illustrates changing power levels prior tothe cumulative thermal dose reaching the thermal dose threshold.

As shown in FIG. 6A, graph 90 illustrates example selected power level92 of charging device 20 for charging rechargeable power source 18. Whencharging is initiated, or started, at the zero minute mark, chargingdevice 20 may select a high power level. The initial high power levelbetween the zero and 20 minute marks may be selected to chargerechargeable power source 18 at a fast rate, e.g., a “boost”. This fastrate may minimize the amount of time patient 12 may need to rechargerechargeable power source 18. Charging device 20 may use the high powerlevel to transmit energy to IMD 14 until the calculated cumulativethermal dose begins to approach the thermal dose threshold.

Charging device 20 may calculate an available thermal dose to determinewhen to select a lower power level for the recharge session. Theavailable thermal dose may be calculated by subtracting the cumulativethermal dose from the thermal dose threshold. Thus, the availablethermal dose may indicate the total heat that IMD 14 can still safelyprovide to surrounding tissue. Charging device 20 may compare theavailable thermal dose to a high power dose requirement to determinewhen to reduce the power from the selected high power level. In thismanner, charging device 20 may select the high power level when theavailable thermal dose is greater than the high power dose requirement,e.g., between the zero and 20 minute mark.

Once charging device 20 calculates that the available thermal dose isless than the high power dose requirement, charging device 20 may selectthe medium power level. Charge level change 94 indicates that the powerlevel was changed from high to medium once the cumulative thermal doseexceeded the high power dose threshold. Then, charging device 20 maycharge rechargeable power source 18 with the medium power level betweenminutes 20 and 25. When the cumulative thermal dose exceeds the thermaldose threshold, charge level change 96 indicates that charging deviceselects the low power level for additional charging of rechargeablepower source 18. Selected power level 92 thus changes as the cumulativethermal dose indicates the amount of heat received by tissue surroundingIMD 14. Charging device 20 may continue to charge rechargeable powersource 18 with the low power level until rechargeable power source 18 isfully charged. Once rechargeable power source 18 is fully charged,charging device 20 may terminate charging by selecting a zero powerlevel.

Reducing the charging power level prior to exceeding the thermal dosethreshold may allow charging device 20 to ensure that the cumulativethermal dose is not exceeded. In other words, IMD 14 may still radiateheat after the charging power has been decreased. By reducing the powerlevel prior to reaching the thermal dose threshold, IMD 14 may radiatethe heat that remains in IMD 14 from the previously higher power level.Once the thermal dose threshold is exceeded, the IMD 14 may have lessresidual heat to dissipate. Therefore, reducing the power level usingthe available thermal dose, as opposed to waiting until the thermal dosethreshold is exceeded, may allow charging device 20 to better controlthe total amount of heat to which tissue is exposed.

As shown in FIG. 6B, graph 98 illustrates charging rate 100 over timedue to varying power levels selected by charging device 20. High rate102 may be representative of the charging rate of rechargeable powersource 18 when charging device 20 selects the high power level forcharging (e.g., the boost rate). Once the cumulative thermal doseexceeds the high power dose requirement, charge rate change 108indicates that the charge rate has been lowered. After charge ratechange 108, the medium power level induces charging rechargeable powersource 18 with medium rate 104. Further, once the cumulative thermaldose exceeds the thermal dose threshold, charge rate change 110indicates that the charge rate has been lowered. After charge ratechange 110, the low power level induces charging rechargeable powersource 18 with low rate 106. Once the charge level for rechargeablepower source 18 reaches approximately 100 percent, the charge rate maybe reduced to zero because the recharge session may be terminated.

Graph 90 of FIG. 6A indicates high, medium, and low power levels. Graph90 indicates that charging device selects between three different powerlevels based on the cumulative thermal dose calculated from the tissuetemperature. In other examples, charging device may utilize a greaternumber of power levels to change the power level in smaller increments.Therefore, charging device 20 may provide finer control of the rechargerate and the temperature of IMD 14 during the charging session. Thefiner control of power may allow charging device 20 to gradually changethe temperature of IMD 14, e.g., reduce the temperature of IMD 14 suchthat the cumulative thermal dose does not exceed the thermal dosethreshold even after charging stops.

In FIGS. 5A, 5B, 6A, and 6B, charging device 20 selects the low chargelevel even after the cumulative thermal dose exceeds the thermal dosethreshold. In these cases, the low charge level may only causenegligible heating of IMD 14. In other words, the heat produced in IMD14 during the corresponding low charge rate may cause an insignificantincrease to the cumulative thermal dose because the temperature issimilar to that of normal body temperature. However, in other example,the low charge level may still generate heat in IMD 14 and contribute tothe cumulative thermal dose. In this case, charging device 20 mayterminate the charging of rechargeable power source 18 (e.g., select azero power level).

FIGS. 7A and 7B are graphs of example selected power levels due to animposed lockout period L after charging with a high power level. Asshown in FIG. 7A, charging device 20 may charge rechargeable powersource 18 with a selected high power level. Charge level 114 of graph112 indicates, however, that the charge level may change as needed tocharge rechargeable power source 18 and limit the cumulative thermaldose delivered to patient 12. Charge level change 116 at t₀ may indicatethat charging was stopped because either the cumulative thermal doseexceeded a threshold or rechargeable power source 18 reached a 100percent charge level.

As shown in the example of FIG. 7A, lockout period L may be implementedafter high power levels to limit the use of the high power level andassociated relatively high temperatures generated in IMD 14 due to thesehigh charge rates. After the power level is reduced to zero at chargelevel change 116, lockout period L may prevent any charging ofrechargeable power source 18 between t₀ and t₁. Once lockout period Lexpires, charging device 20 may again select a power level to chargerechargeable power source 18, such as the high power level indicatedafter charge level change 118. Although charging device 20 may begincharging rechargeable power source 18 immediately after lockout period Lexpires, charging is not required to begin immediately. Lockout period Lmerely enables charging after expiration.

Graph 120 provides charge level 122 varying in response to lockoutperiod H. In the example of FIG. 7B, lockout period H may also beimplemented after high power levels to limit the use of the high powerlevel and associated relatively high temperatures generated in IMD 14due to these high charge rates. After the power level is reduced to zeroat charge level change 124, lockout period H may prevent only theselection of high power for charging. Therefore, lockout period H mayprevent charging of rechargeable power source 18 with the high powerlevel between t₀ and t₂. However, lockout period H may not preventcharging device 20 from charging rechargeable power source 18 with a lowpower level during the lockout period. During lockout period H, chargingdevice 20 may select the low power level at charge level change 126.This low power level selection may allow rechargeable power source 18 tobe charged slowly because IMD 14 may not radiate any significant amountof heat during this slow charge.

Once lockout period H expires, charging device 20 may again select apower level to charge rechargeable power source 18, such as the highpower level indicated after charge level change 128. Although chargingdevice 20 may begin charging rechargeable power source 18 with the highpower level immediately after lockout period H expires, the high powerlevel is merely allowed to be selected any time after the expiration oflockout period H. In some examples, the medium power level, or otherpower levels, may also be selected during lockout period H. These lowerpower levels may not significantly increase the temperature of IMD 14and surrounding tissue while allowing rechargeable power source 18 tostill be replenished. Since the high power level may be considered a“boost” in charging in some examples, the high power level may not beneeded to recharge rechargeable power source 18.

The duration of the lockout period may vary in different examples andfor different purposes. For example, the lockout period may be set to apredetermined period of time that simply limits the frequency of thehigh power use. This predetermined period may be between approximately10 minutes and 48 hours. In another example, the lockout period maychange based upon the difference between the available thermal dose andthe high power dose requirement. Charging device 20 may calculate thedifference between the cumulative thermal dose and the thermal dosethreshold to determine the available thermal dose. If the high powerdose requirement (e.g., the thermal dose required to charge rechargeablepower source 18 with the high power level) is greater than the availablethermal dose, then the lockout period continues. In this manner, thelockout period may not be a timer that simply allows charging once itexpires. In other words, the lockout period may be based on the amountof time charging occurred with the high power level as a proxy for theactual cumulative thermal dose received by patient 12 during the highpower level. Alternatively, the lockout period may be based directly oncalculated cumulative thermal dose.

In other examples, the lockout period may be initiated only after thecumulative thermal dose has exceeded the thermal dose threshold. SinceIMD 14 may continue to expose tissue to heat even after charging stopswhen the cumulative thermal dose has exceeded the thermal dosethreshold, the lockout period may be started to allow the surroundingtissue to recover from the charging session. The lockout period mayprevent any charging from occurring or, in some examples, allow chargingdevice 20 to select a low power level for charging with minimal heatgeneration. Alternatively, charging device 20 may implement multiplelockout period. For example, a full lockout period may prevent anycharging from occurring while a high power lockout period may onlyprevent charging device 20 from selecting the high power level. Thesemultiple timers may operate simultaneously.

FIG. 8 is a flow diagram that illustrates an example technique forselecting a power level for charging implantable rechargeable powersource 18 based on a calculated cumulative thermal dose. Althoughprocessor 50 of charging device 20 will be described as generallyperforming the technique of FIG. 8, the technique of FIG. 8 may insteadbe performed by a combination of processors 30 and 50, in otherexamples.

A charging session for rechargeable power source 18 may begin whenprocessor 50 receives a charge request via user interface 54 (130).Processor 50 may calculate the cumulative thermal dose to verify howmuch heat tissue surrounding IMD 14 has been exposed to recently (132).When processor 50 calculates the tissue temperature using transmittedpower, processor 50 may calculate the cumulative thermal dose withoutdata from IMD 14. When processor 50 calculates the tissue temperatureusing power measured in IMD 14 or temperatures measured at IMD 14,processor 50 may incorporate the appropriate data received from IMD 14.As described herein, the cumulative thermal dose may be calculated basedupon tissue temperatures over a period of time. Since the period of timemay be a rolling period of time, the cumulative thermal dose maydecrease with time as rechargeable power source 18 is not being charged.

If the cumulative thermal dose is less than the thermal dose threshold(“NO” branch of block 134), processor 50 selects the high power levelfor charging (140). If the cumulative thermal dose is equal to orgreater than the thermal dose threshold (“YES” branch of block 134),processor 50 selects the low power level for charging (136). Ifprocessor 50 is switching to a low power level from a high power level,user interface 54 may notify the user via a sound or visual indicationthat such change has occurred. Processor 50 then instructs chargingmodule 58 to charge rechargeable power source 18 with the selected powerlevel (138). In alternative examples, processor 30 may calculate thecumulative thermal dose and or select the power level for charging. Inthese examples, processor 50 may incorporate this information receivedfrom IMD 14 to perform at least some of the elements of FIG. 8.

If rechargeable power source 18 has not yet reached a 100 percent, orfull, charge level (“NO” branch of block 142), then processor 50continues to calculate the cumulative thermal dose (132). Ifrechargeable power source 18 has reached a 100 percent, or full, chargelevel (“YES” branch of block 142), then processor 50 may instructcharging module 58 to terminate charging (144). In other words,processor 50 may select a zero power level. Charging device 20 maysubsequently notify the user of the completed recharge of rechargeablepower source 18 and IMD 14 (146). This notification may be in the formof an audible alert or visual indicator provided by user interface 54.Processor 50 may also terminate charging upon request from the user.

In alternative examples, processor 50 may not charge rechargeable powersource 18 when the cumulative thermal dose meets or exceeds the thermaldose threshold. Therefore, not even a low power level would be selected.The ability to charge rechargeable power source 18 at any power levelafter the cumulative thermal dose has exceeded the thermal dosethreshold may be dependent upon how much heat is generated in IMD 14when various power levels are used to charge rechargeable power source18. Although a low power level may be acceptable for charging at anytime in some systems and patients, other systems may be programmed tonot allow any charging after the thermal dose threshold is exceeded.

FIG. 9 is a flow diagram that illustrates an example technique forselecting a power level for charging implantable rechargeable powersource 18 based on an available cumulative thermal dose remaining forthe charging process. The available thermal dose may allow chargingpower levels to be reduced prior to exceeding the thermal dosethreshold. Although processor 50 of charging device 20 will be describedas performing the technique of FIG. 9, the technique of FIG. 9 mayinstead be performed by processor 30 of IMD 14, or a combination ofprocessors 30 and 50, in other examples.

A charging session for rechargeable power source 18 may begin whenprocessor 50 receives a charge request via user interface 54 (150).Processor 50 may calculate the cumulative thermal dose to verify howmuch heat tissue surrounding IMD 14 has been exposed to recently (152).If the cumulative thermal dose is less than the thermal dose threshold(“NO” branch of block 154), processor 50 calculates the availablethermal dose (160). If the available thermal dose is greater than thehigh power dose requirement (“YES” branch of block 162), processor 50selects the high power level for charging (166). If the availablethermal dose is less than the high power dose requirement (“NO” branchof block 162), processor 50 selects the medium power level for charging(164). The medium power level may allow IMD 14 to lower its temperature,and the lower the rate at which the cumulative thermal rate increases,while still charging rechargeable power source 18.

If the cumulative thermal dose is equal to or greater than the thermaldose threshold (“YES” branch of block 154), processor 50 selects the lowpower level for charging (156). If processor 50 is switching to adifferent power level, user interface 54 may notify the user via a soundor visual indication that such change has occurred. After the selectionof the appropriate power level, processor 50 then instructs chargingmodule 58 to charge rechargeable power source 18 with the selected powerlevel (158). This technique for selecting power levels for chargingrechargeable power source 18 and IMD 14 may allow processor 50 to limitheat radiated by IMD 14 after the thermal dose threshold has beenexceeded.

If rechargeable power source 18 has not yet reached a 100 percent, orfull, charge level (“NO” branch of block 168), then processor 50continues to calculate the cumulative thermal dose (152). Ifrechargeable power source 18 has reached a 100 percent, or full, chargelevel (“YES” branch of block 168), then processor 50 may instructcharging module 58 to terminate charging and notify the user of thetermination (170). This notification may be in the form of an audiblealert or visual indicator provided by user interface 54. Processor 50may also terminate charging upon request from the user.

FIG. 10 is a flow diagram that illustrates an example technique forimplementing a lockout period for high power level charging of arechargeable power source. The lockout period may prevent chargingdevice 20 from charging IMD 14 and exposing surrounding tissue tounacceptable temperatures. Although processor 50 of charging device 20will be described as performing the technique of FIG. 10, the techniqueof FIG. 10 may instead be performed by processor 30 of IMD 14, or acombination of processors 30 and 50, in other examples.

A charging session for rechargeable power source 18 may begin whenprocessor 50 receives a charge request via user interface 54 (172). Ifprocessor 50 determines that the lockout period has not yet expired(“YES” branch of block 174), the processor 50 may select the low powerlevel for charging (184). If processor 50 determines that the lockoutperiod has expired (“NO” branch of block 174), then processor 50 maycalculate the cumulative thermal dose to verify how much heat tissuesurrounding IMD 14 has been exposed to recently (176). If the cumulativethermal dose is less than the thermal dose threshold (“NO” branch ofblock 178), processor 50 selects the high power level for charging(186).

If the cumulative thermal dose is equal to or greater than the thermaldose threshold (“YES” branch of block 178), processor 50 determines ifthe current selected power level is the high power level (180). If thehigh power level is currently selected (“YES” branch of block 180),processor 50 starts the lockout period (182) and selects the low powerlevel for charging (184). If the high power level is not currentlyselected (“NO” branch of block 180), then processor 50 selects the lowpower level (184). In this manner, processor 50 begins or initiates thelockout period upon switching from the high power level. The lockoutperiod thus prevents processor 50 from selecting the high power levelagain until the lockout period has expired (174). After the selection ofthe appropriate power level, processor 50 then instructs charging module58 to charge rechargeable power source 18 with the selected power level(188).

If rechargeable power source 18 has not yet reached a 100 percent, orfull, charge level (“NO” branch of block 190), then processor 50continues to calculate the cumulative thermal dose (176). Ifrechargeable power source 18 has reached a 100 percent, or full, chargelevel (“YES” branch of block 168), then processor 50 may instructcharging module 58 to terminate charging and notify the user of thetermination (192). This notification may be in the form of an audiblealert or visual indicator provided by user interface 54. Processor 50may also terminate charging upon request from the user.

In the example of FIG. 10, the lockout period may be a predeterminedperiod regardless of the amount of time the high power level was used tocharge rechargeable power source 18 or the cumulative thermal dose. Insome examples, processor may calculate the lockout period duration whenthe lockout period is started. The lockout period may be based on theduration the high power level was used to charge rechargeable powersource 18, e.g., longer high power level charging would result in alonger lockout period. In other examples, the lockout period may reflectthe cumulative thermal dose such that the lockout period may expire uponthe cumulative thermal dose dropping below a threshold. In this manner,processor 50 may calculate the cumulative thermal dose prior todetermining if the lockout period has expired. In any case, the lockoutperiod may be implemented by charging device 20 or IMD 14 to preventexcessive IMD 14 temperatures from higher power levels, or highercharging rates, during a recharge session.

According to the techniques and devices described herein, a cumulativethermal dose may be calculated as feedback for the selection of a powerlevel used to charge a rechargeable power source in an IMD. The powerlevel may be reduced upon the cumulative thermal dose exceeding athreshold to limit the risk of undesirable and uncomfortable higher IMDtemperatures during recharging. The cumulative thermal dose may alsoallow for real-time feedback that allows a charging device to chargingthe rechargeable power source at high rates in a “boost mode” and lowerthe charging rate before the temperature may be damaging to tissue. Inthis manner, the charging device or IMD may balance the desire for fastcharging rates with patient safety without estimations used in open-loopcharging techniques.

This disclosure is primary directed to wireless transfer of energybetween two coils (e.g., inductive coupling). However, one or moreaspects of this disclosure may also be applicable to energy transferinvolving a physical connection between a charging device and arechargeable power supply. For example, aspects of this disclosure maybe applicable to charging the power supply of an IMD by inserting aneedle coupled to an external charging device through the skin and intoa port of the IMD. Although physical connections for energy transfer maynot introduce heat losses due to energy transfer between wireless coils,heat may still be generated and lost to the patient from componentswithin the IMD (e.g., the battery being charged and circuits involved inthe recharging of the power supply).

Various examples have been described. These and other examples arewithin the scope of the following claims.

1. A method comprising: calculating, by a processor, an estimatedcumulative thermal dose delivered to a patient during charging of arechargeable power source of an implantable medical device over a periodof time; and selecting, by the processor, a power level for subsequentcharging of the rechargeable power source based on the estimatedcumulative thermal dose.
 2. The method of claim 1, further comprisingcharging the rechargeable power source with the selected power level. 3.The method of claim 2, wherein charging the rechargeable power sourcecomprises: generating a first electrical current in a primary coil of acharging device based on the selected power level; and inducing anelectrical current in an implanted secondary coil associated with therechargeable power source.
 4. The method of claim 1, wherein the periodof time is one of selected from between approximately one hour andforty-eight hours or a single recharge session.
 5. The method of claim1, wherein selecting the power level comprises: selecting a high powerlevel when the estimated cumulative thermal dose has not exceeded athermal dose threshold; and selecting a low power level when theestimated cumulative thermal dose has exceeded the thermal dosethreshold.
 6. The method of claim 1, wherein selecting the power levelcomprises: calculating an available thermal dose by subtracting theestimated cumulative thermal dose from a thermal dose threshold;selecting a high power level when the available thermal dose is greaterthan a high power dose requirement; and selecting a low power level whenthe available thermal dose is less than the high power dose requirement.7. The method of claim 1, further comprising: initiating a lockoutperiod after charging the rechargeable power source with a high powerlevel; and controlling a charging module to prevent selection of thehigh power level during the lockout period, wherein a duration of thelockout period is based on a previous charging time with the high powerlevel.
 8. The method of claim 1, wherein calculating the estimatedcumulative thermal dose comprises integrating a tissue temperature overthe period of time.
 9. The method of claim 8, further comprisingmeasuring the tissue temperature adjacent to the rechargeable powersource.
 10. The method of claim 8, further comprising calculating thetissue temperature based on a tissue model and power transmitted to therechargeable power source over the period of time.
 11. The method ofclaim 1, wherein the processor is within one of the implantable medicaldevice or an external charging device.
 12. A device comprising: aprocessor configured to calculate an estimated cumulative thermal dosedelivered to a patient during charging of a rechargeable power source ofan implantable medical device over a period of time and select a powerlevel for subsequent charging of the rechargeable power source based onthe estimated cumulative thermal dose.
 13. The device of claim 12,further comprising a charging module configured to charge therechargeable power source with the selected power level.
 14. The deviceof claim 13, further comprising a primary coil, wherein the chargingmodule is configured to generate a first electrical current in theprimary coil based on the selected power level, and wherein the primarycoil induces an electrical current in an implanted secondary coilassociated with the rechargeable power source.
 15. The device of claim12, wherein the period of time is one of selected from betweenapproximately one hour and forty-eight hours or a single rechargesession.
 16. The device of claim 12, wherein the processor is configuredto: select a high power level when the estimated cumulative thermal dosehas not exceeded a thermal dose threshold; and select a low power levelwhen the estimated cumulative thermal dose has exceeded the thermal dosethreshold.
 17. The device of claim 12, wherein the processor isconfigured to: calculate an available thermal dose by subtracting theestimated cumulative thermal dose from a thermal dose threshold; selecta high power level when the available thermal dose is greater than ahigh power dose requirement; and select a low power level when theavailable thermal dose is less than the high power dose requirement. 18.The device of claim 12, wherein the processor is configured to initiatea lockout period after the rechargeable power source is charged with ahigh power level and control a charging module to prevent selection ofthe high power level during the lockout period, wherein a duration ofthe lockout period is based on a previous charging time with the highpower level.
 19. The device of claim 12, wherein the processor isconfigured to calculate the estimated cumulative thermal dose byintegrating a tissue temperature over the period of time.
 20. The deviceof claim 19, further comprising a telemetry module configured to receivea tissue temperature from the implantable medical device, wherein thetissue temperature is measured adjacent to the rechargeable powersource.
 21. The device of claim 19, further comprising a memoryconfigured to store a tissue model, wherein the processor is configuredto calculate the tissue temperature based on the tissue model and powertransmitted to the rechargeable power source over the period of time.22. The device of claim 12, wherein the processor is contained withinone of the implantable medical device or a charging device configured tocharge the rechargeable power source.
 23. A computer-readable storagemedium comprising instructions that cause at least one processor to:calculate an estimated cumulative thermal dose delivered to a patientduring charging of a rechargeable power source of an implantable medicaldevice over a period of time; and select a power level for subsequentcharging of the rechargeable power source based on the estimatedcumulative thermal dose.
 24. The computer-readable storage medium ofclaim 23, further comprising instructions that cause the at least oneprocessor to charge the rechargeable power source with the selectedpower level.
 25. The computer-readable storage medium of claim 23,wherein the instructions that cause the at least one processor to selectthe power level comprises instructions that cause the at least oneprocessor to: select a high power level when the estimated cumulativethermal dose has not exceeded a thermal dose threshold; and select a lowpower level when the estimated cumulative thermal dose has exceeded thethermal dose threshold.
 26. A system comprising: an implantable medicaldevice comprising a rechargeable power source; an external chargingdevice comprising a charging module configured to charge therechargeable power source; and at least one processor configured tocalculate an estimated cumulative thermal dose delivered to a patientduring charging of the rechargeable power source over a period of timeand select a power level for subsequent charging of the rechargeablepower source based on the estimated cumulative thermal dose.
 27. Thesystem of claim 26, wherein: the external charging device comprises aprimary coil; the implantable medical device comprises a secondary coilcoupled to the rechargeable power source; the charging module isconfigured to generate a first electrical current in the primary coilbased on the selected power level; and the primary coil induces anelectrical current in the secondary coil of the implantable medicaldevice used to charge the rechargeable power source.
 28. The system ofclaim 26, wherein the external charging device comprises the at leastone processor.
 29. The system of claim 26, wherein the implantablemedical device comprises the at least one processor.
 30. The system ofclaim 26, wherein the at least one processor comprises a first processorand a second processor, and wherein: the implantable medical devicecomprises the first processor configured to calculate the estimatedcumulative thermal dose delivered to the patient during charging of therechargeable power source over the period of time; and the externalcharging device comprises the second processor configured to select apower level for subsequent charging of the rechargeable power sourcebased on the estimated cumulative thermal dose.
 31. The system of claim30, wherein: the implantable medical device comprises a first telemetrymodule configured to transmit the estimated cumulative thermal dose tothe external charging device; and the external charging device comprisesa second telemetry module configured to receive the estimated cumulativethermal dose from the first telemetry module.